1. Field of the Invention
The present invention relates to a calibration circuit and method for a maximum and minimum value detection apparatus that detects maximum and minimum values of an input signal.
2. Description of the Related Art
Maximum and minimum value detection apparatuses are known to be used in digital oscilloscopes or the like.
A digital oscilloscope displays an input signal after converting the input signal to a digital signal by sampling the input signal. Consequently, when the input signal includes frequency components higher than the Nyquist frequency or half of the sampling frequency, so-called aliasing arises. A maximum and minimum value detection apparatus has been employed to prevent adverse effects by such an aliasing phenomenon, or to display envelopes of the input signal.
Maximum and minimum value detection apparatuses are roughly divided into two classes: one processing an input signal with a digital circuit; and the other processing an input signal with an analog circuit.
A digital type maximum and minimum value detection apparatus, which processes an input signal by using a digital circuit, samples an input signal at a sampling rate which is higher than the standard sampling rate at which the input signal is sampled, and then sequentially compares the sampled data. The maximum value detection apparatus holds the greatest value sampled over a predetermined interval and the minimum value detection apparatus holds the lowest value sampled over that interval. This process is performed over several predetermined detection intervals, thus yielding the maximum value and the minimum value for each of those intervals.
The digital type maximum and minimum value detection apparatus has a problem in that a maximum or minimum value of an interval between two adjacent sampling points cannot be detected because the sampling is a discrete operation, and hence does not continually detect the input signal during the interval.
On the other hand, an analog type maximum and minimum value detection apparatus that processes the input signal with an analog circuit can continually detect a maximum or minimum value in the predetermined detection interval because it operates continuously even between the sampling points of a digital oscilloscope.
FIG. 1 is a block diagram showing an arrangement of the analog type maximum and minimum value detection apparatus. In FIG. 1, reference numeral 71 designates a maximum value detection circuit; 72, a minimum value detection circuit; and 73, a preset adjuster for manually adjusting an offset of the minimum value detection circuit 72.
The maximum value detection circuit 31 has an arrangement as shown in FIG. 2. In FIG. 2, a charging amplifier 1 charges a capacitor 2 through a diode 3. The capacitor 2 is discharged through a resistor 4 and a switch 5 is closed by a reset signal. The output of the capacitor 2 is produced via a buffer amplifier 6.
On the other hand, the minimum value detection circuit 72 has an arrangement as shown in FIG. 3. The corresponding parts in FIGS. 2 and 3 are designated by the same reference numerals. The minimum value detection circuit 72 differs from the maximum value detection circuit 71 in that a diode 7 is connected in the direction opposite to that of the diode 3, and that a voltage outputted from a preset adjuster 73 is applied to an offset voltage adjusting terminal of the buffer amplifier 6. Furthermore, a charging amplifier 8 of the minimum value detection circuit 72 uses opposite conductivity type semiconductors to those of the charging amplifier 1 of the maximum detection circuit 71, and the polarity of power supplies of the circuits 71 and 72 are also opposite in polarity.
In the maximum value detection circuit 71 thus constructed, the capacitor 2 is charged up to a maximum value of an input signal by the charging amplifier 1, and the voltage of the capacitor 2 is outputted from the buffer amplifier 6. In contrast, in the minimum value detection circuit 72, the capacitor 2 is charged to a minimum value of an input signal, and the voltage of the capacitor 2 is outputted from the buffer amplifier 6. In a calibration mode, a signal of a predetermined level is applied by an operator to the inputs of the maximum value detection circuit 71 and the minimum value detection circuit 72, and the offset voltage of the minimum value detection circuit 72 is adjusted by an operator's manipulation of the preset adjuster 43 so that the output levels of the two circuits 71 and 72 become identical.
The maximum value detection circuit 71 and the minimum value detection circuit 72 which process analog signals have their peculiar output offset voltages which are liable to be varied by ambient conditions or the like. Accordingly, even when a DC voltage applied by an operator to the circuits 71 and 72 is maintained at a fixed value, it is very probable that the outputs of these circuits will become different over time.
Thus, when the maximum value detection circuit and the minimum value detection circuit 72 with the preset adjuster 73 are constructed from commercially available discrete parts, the following problem arises: the output offset voltages of the circuits 71 and 72 change independently owing to temperature variation or deterioration with age, and hence, it is unavoidable that errors are included in observation results even when errors of the output offset voltages of the circuits 71 and 72 have been initially canceled by operator adjustment of the preset adjuster 33.
FIG. 4 shows another technique to reduce the errors of the maximum and minimum value detection apparatus. The detection apparatus includes a differential amplifier 83, and two identically arranged maximum value detection circuits 81 and 82. The noninverted output and inverted output of the differential amplifier 83 are fed to the maximum value detection circuits 81 and 82, respectively. This makes it possible to reduce the difference between the characteristics of the two circuits 81 and 82. Here, the minimum value of the input signal becomes the maximum value at the inverted side, and hence, the output of the maximum value detection circuit 82 produces the minimum value of the input signal.
Furthermore, to eliminate errors of a maximum value detection circuit and a minimum value detection circuit, characteristics of semiconductor devices making up the circuits are matched by the following technique: the circuits are arranged on the same semiconductor pellet so as to integrate them into an IC; or the circuits are arranged by pair semiconductor devices having matched characteristics.
The conventional techniques to reduce the errors have a problem that the circuits become expensive compared with the circuits composed of commercially available discrete parts.